An integer array is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.
For example, [1,3,5,7,9], [7,7,7,7], and [3,-1,-5,-9] are arithmetic sequences.Given an integer array nums, return __the number of arithmetic subarrays of__ nums.
A subarray is a contiguous subsequence of the array.
Example 1: Input: nums = [1,2,3,4] Output: 3 Explanation: We have 3 arithmetic slices in nums: [1, 2, 3], [2, 3, 4] and [1,2,3,4] itself. Example 2: Input: nums = [1] Output: 0
Constraints:
1 <= nums.length <= 5000
-1000 <= nums[i] <= 1000
- code
# [1,3,5,7,9,9,9] ; 1,3,5; 1,3,5,7; 3,5,7; 3579; 579; 999;
# 13579; 999 6+1 7
# 1 + 2 + 3.. (n-3+1)
# (1 + (n - 3 + 1)) * (n-3+1) // 2
# length of consecutive elements -> num of subarrays (n - 1)(n - 2) // 2
# 9999 999, 999 9999
class Solution:
def numberOfArithmeticSlices(self, nums: List[int]) -> int:
if len(nums) < 3: return 0
nums.append(inf)
diff = nums[1] - nums[0]
i = 2
cur_len = 2
res = 0
while i < len(nums):
if nums[i] - nums[i-1] == diff:
cur_len += 1
else:
res += (cur_len - 1) * (cur_len - 2) // 2
diff = nums[i] - nums[i-1]
cur_len = 2
i += 1
return res